1. Field of the Invention
The present invention relates to a method for protecting dies during a die closing operation in an injection molding machine and, more specifically, to a method for monitoring a forward thrust of a movable die.
2. Description of the Related Art
In a prior art injection molding machine, when a die closing operation is performed, a forward thrust of a movable platen is monitored. When the forward thrust exceeds a preset upper limit, the machine determines that some abnormality has occurred and protects dies. More specifically, a specific monitoring section is set within a moving range of the movable platen immediately before a movable die is brought into close contact with a stationary die. When the forward thrust of the movable platen exceeds the above upper limit within the monitoring section, the machine determines that some abnormality has occurred and stops the movable platen from moving forward.
In the prior art injection molding machine, the monitoring section of the forward thrust of the movable platen is limited to a range immediately before the movable die and the stationary die are brought into close contact with each other. In the case of dies having a plurality of plates, if the movable platen falls outside the monitoring section, abnormalities (e.g., galling of a sliding section of a slide core, shortage of lubricating oil, galling of another sliding section) cannot be detected during the die closing operation.
FIGS. 8 and 9 each show an example of a die closing operation in dies having a plurality of plates.
When a die closing operation is normally performed, as shown in FIG. 8, an angular pin 6, which is provided on the front of a movable plate 1A that is part of a stationary die 1, is inserted into an inclined hole of a slide core 7 provided on the front of a movable die 3, and the slide core 7 is pressed and moved by the angular pin 6, with the result that the front of a movable die 2 is brought into close contact with that of the movable plate 1A. After that, while the movable die 2 remains in contact with the movable plate 1A, it continues to advance and stops when the back of the movable plate 1A is brought into contact with a runner stripper plate 1B that is the remaining part of the stationary die 1.
In contrast, when a die closing operation is performed while the slide core 7 is not located in a normal position, as shown in FIG. 9, the angular pin 6 is not inserted into the inclined hole of the slide core 7 but the movable die 2 advances while the top end of the angular pin 6 is in contact with the vicinity of the edge of the slide core 7. In this case, even though the back of the movable plate 1A contacts the runner stripper plate 1B, the die closing operation is continued since the movable die 2 has not yet reached a die closing completion position. Consequently, the forward thrust of a movable platen 4 is all exerted on the angular pin 6 to cause damage to the angular pin 6.
If, in the above case, a monitoring section of the forward thrust of the movable platen 4 is set longer than the length of the angular pin 6, a die protecting operation can be carried out by detecting that the forward thrust exceeds the upper limit. For the die protecting operation, it is preferable that the upper limit be set as low as possible. The above case, however, requires the minimum thrust for inserting the angular pin 6 into the inclined hole of the slide core 7, and the set upper limit becomes larger than a normal one. Since the forward thrust corresponding to the larger upper limit is applied to the angular pin 6, the possibility that the angular pin 6 will be damaged is not eliminated. If the upper limit is set larger than the normal one, the same problem will occur even though products are left in the die due to a drop failure and a take-out failure. In other words, the forward thrust of the movable platen is exerted on the products remaining between the dies in the monitoring section, thereby causing damage to the dies.
Moreover, in the monitoring section of the forward thrust, the die closing speed usually needs to be set lower in order to protect the dies effectively. If, therefore, the monitoring section is lengthened, cycle time for injection molding is lengthened to decrease productivity.
The present invention has been developed in consideration of the above problems of the prior art method for protecting dies in an injection molding machine. An object of the present invention is to provide a method for protecting dies, which is capable of reliably detecting abnormalities during a die closing operation and hardly decreases productivity.
According to an aspect of the present invention, there is provided a method for protecting dies in an injection molding machine, the injection molding machine comprising:
a stationary platen which holds a stationary die;
a movable platen arranged opposite to the stationary platen, which holds a movable die; and
a driving control system which drives the movable platen and performs a die closing operation between the movable die and the stationary die,
wherein the method comprises:
setting a plurality of monitoring sections within a moving range of the movable platen;
setting an upper limit of forward thrust of the movable platen for each of the monitoring sections; and
performing a die protecting operation when the forward thrust of the movable platen reaches the upper limit set in one of the monitoring sections during the die closing operation.
According to the die protecting method described above, a plurality of monitoring sections are set within a moving range of a movable platen in accordance with the positions of respective guide members provided on the contact surface of a die and the contents of abnormalities that are expected during the die closing operation, and an upper limit of the forward thrust of the movable platen is set for each of the monitoring sections. The abnormalities can thus be detected with reliability during the die closing operation and the possibility that the dies will be damaged can be reduced.
Furthermore, cycle time for molding can be minimized by performing a monitoring operation only in the section where the occurrence of abnormalities is expected during the die closing operation.
Preferably, the driving control system comprises a servomotor whose torque and speed is controlled, and a ball screw driven by the servomotor. The ball screw moves the movable platen.
Preferably, the driving control system comprises a hydraulic cylinder and a hydraulic circuit which controls pressure and a flow rate of oil supplied to the hydraulic cylinder. The hydraulic cylinder moves the movable platen.
In an injection molding machine having a toggle link mechanism for die closing, usually, the forward thrust of the movable platen cannot directly be detected. It is thus necessary to calculate the forward thrust of the movable platen from the thrust of a driving source of the toggle link mechanism.
If the driving control system comprises a link housing, a toggle link mechanism which couples the link housing and the movable platen to each other, and a crosshead which expands or contracts the toggle link mechanism and advances or retreats the movable platen, the forward thrust of the movable platen can be calculated from the thrust of the crosshead by the following method. A table representing a correlation between a position of the crosshead and a position of the movable platen is stored in advance in a storage unit. A speed ratio between the crosshead and the movable platen is calculated from an amount of movement of the crosshead per unit time and that of movement of the movable platen per unit time using the table during a die closing operation. The forward thrust of the movable platen can thus be calculated from the thrust of the crosshead using the speed ratio.